1. Field of the Invention
The present invention relates to an acceleration sensor capable of performing sensitivity adjustment.
2. Description of the Related Art
Conventionally, as an acceleration sensor using piezoelectric ceramics, for example, the sensor described in Japanese Patent No. 2780594 has been known. Since such a sensor does not have an adjustment method for adjusting an individual sensor to a targeted sensitivity (voltage output in the case of applying a specified acceleration) or has difficulty in adjusting an individual one to a targeted sensitivity, it has a sensitivity tolerance between approximately xc2x15 and xc2x115% due to variations in manufacturing and materials.
Meanwhile, a sensor with high precision, which has a sensitivity tolerance less than xc2x15%, such as an acceleration sensor used for servo control of a MR-head for Hard Disc Drive, is sometimes required. In order to meet such a demand, it is an ordinary procedure that a gain of an amplifier connected to the subsequent stage of the sensor is adjusted. As a result, in the conventional art, the only way to adjust the sensitivity of a sensor is to connect the sensor to the amplifier and change a resistance value of a gain adjustment resistor.
Accordingly, it is an object of the present invention to provide an acceleration sensor with high precision, which can perform sensitivity adjustment by the sensor alone.
In order to achieve the above object, the present invention provides an acceleration sensor comprising: an acceleration sensing device; an insulating case including the acceleration sensing device; and at least one sensitivity adjustment trimmable resistor provided on a surface of the insulating case and connected to the acceleration sensing device.
In accordance with one aspect of the present invention, two external lead electrodes are connected to the acceleration sensing device, and another external lead electrode, which is different from the external electrodes connected to the acceleration sensing device, is connected to one end of the trimmable resistor, all of the external leads being provided on the surface of the insulating case.
The trimmable resistor is disposed on a surface of the insulating case of the acceleration sensor and at least one end of the trimmable resistor is extended outside through the external lead electrode which is different from the external lead electrodes of the acceleration sensing device. The other end of the trimmable resistor may also be extended outside through the external lead electrode, which is different from the external lead electrodes of the acceleration sensing device.
The resistance value of the trimmable resistor is adjusted by laser trimming or the like so as to freely adjust the sensitivity of the acceleration sensor and to allow variations in the sensitivity to be held within a range of a targeted tolerance. Since the trimmable resistor is integrally disposed in the acceleration sensor, sensitivity adjustment can be performed by the sensor alone so that a high-precision acceleration sensor can be obtained.
According to another aspect of the present invention, when the other end of the trimmable resistor is connected to an external lead electrode of the acceleration sensor in common, the number of the external lead electrodes formed on the insulated case can be reduced; thereby cost reduction can be achieved.
The trimmable resistor may be, for example, a discrete component mounted by soldering or other ways on the insulating case of the acceleration sensor. However, as another aspect of the present invention it is preferable that a film-formed resistor which is directly and integrally mounted on the insulated case by printing method or the like is used. In this case, modifications in the shape of the package of the acceleration sensor are not necessary, and miniaturization can be achieved.
According to another aspect of the present invention, two sensitivity adjustment trimmable resistors are disposed on the surface of the insulating case; both ends of the first trimmable resistor are connected respectively to external lead electrodes formed on the insulating case which are different from the external lead electrodes of the acceleration sensing device; and the second trimmable resistor is connected between one of the external lead electrodes of the acceleration sensing device and one of the external lead electrodes connected to the first trimmable resistor. For example, when an amplifier is connected to the subsequent stage of the acceleration sensor, the first trimmable resistor formed on the insulating case of the acceleration sensor can be used as a feedback resistor of the amplifier, so that a gain of the amplifier can be freely adjusted.
Although, a sensor using a semiconductor as an acceleration sensor may be used, as another aspect of the present invention, it is preferable that a piezoelectric acceleration sensor, in which a piezoelectric ceramic device converts an acceleration into electrical signals is used. In this case, the insulating case can be formed of an insulating material such as a ceramic.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.